It's happened to me a few times with guest shooters at the range - and it certainly isn't the end of the world. I've successfully used a piece of stiff metal measuring tape to shim the mag out - basically required if the hammer is cocked. You can also buy the "Mag out" tool online, which is a little easier to use and does basically the same thing. I can describe the process in more detail if you need, or you can just search around the forum some - Seems to come up every few months.
If the hammer is not cocked and you can get the mainspring housing out, there is a piece you can push on with a screwdriver and the mag dropped right out for me.
As for the tool marks on the gun - up to you whether or not they are something you're willing to overlook. If the price is right and you're just looking for a (great) shooter, maybe it's worth it.